1. Field of the Invention
The present invention relates to semiconductor devices and electronic devices.
2. Description of the Related Art
In recent years, electronic circuits (hereinafter referred to as “functional circuits”) which are formed over substrates and have specific functions have been utilized for a variety of electronic components or semiconductor devices such as electronic devices.
As functional circuits, photoelectric conversion circuits and the like can be given, for example. A number of semiconductor devices including photoelectric conversion circuits generally used for detecting an electromagnetic wave (also referred to as the “photoelectric conversion devices”) are known, and for example, a semiconductor device having sensitivity to ultra-violet rays to infrared rays is collectively referred to as a “light sensor”. A light sensor having sensitivity to light in a visible light region with a wavelength of 400 nm to 700 nm is particularly referred to as a visible light sensor, and a large number of visible light sensors are used for devices which require illuminance adjustment, on or off control, or the like depending on human living environment.
A semiconductor device having the aforementioned functional circuit has a problem in that elements included in the circuit are electrically damaged when excessively high voltage is supplied from the outside. For example, there is a problem in that when high voltage of about several kilovolts is supplied to the functional circuit by static electricity or the like, the breakdown due to electrostatic discharge (ESD) occurs in the functional circuit. In this specification, voltage of which the absolute value is so large that elements are damaged is referred to as an “overvoltage”.
In order to solve the aforementioned problems, a semiconductor device is proposed in which an overvoltage protection circuit is provided to suppress the breakdown of elements in a functional circuit even when the overvoltage is applied to the functional circuit from the outside (e.g., Patent Document 1).
For example, in a conventional semiconductor device described in Patent Document 1, diodes are placed into the conductive state when the overvoltage is applied so that tolerance to the overvoltage in an internal circuit can be increased.